This project concerns the measurement and modeling of fields produced by stimulating electrodes to provide a basis for predicting and manipulating excitation by cochlear prostheses. It is funded by grants and contracts other than the Center grant but in many cases uses technology provided through the Center. It provides a collaborative interface for the application of very small, multicontact electrodes, including thin-film implementations, to these field estimations. In the past year measurements of potential fields in the cochlea and in the cochlear nuclei have been made using thin-film and wire-based electrodes. Claude Jolly, in collaboration with Dr. Spelman, has made very high spatial resolution field measurements near new stimulating electrode arrays in saline to obtain precise field profiles for monopolar, bipolar, and quadrupolar stimulation with 10 micron sampling. These measurements have been compared to simple field models as well as lumped-element models of fields generated in cochlear scalae. In the coming year continuing measurements and model predictions will be applied to understanding current spread in the cochlea for a range of electrode configurations. Dr. Clopton has continued field measurements in the cochlear nucleus. Using electrical stimulation of the cochlea, thin-film electrodes were advanced across the genotypic organization of the ventral cochlear nucleus to map excitation patterns produced by monopolar, bipolar, and quadrupolar configurations of scalar electrodes. The distinctive spatial details of excitation for each of these electrode driving schemes were evident from this mapping procedure.